Variable impedance device



Oct. 23, 195] D. F. WALKER VARIABLE IMPEDANCE DEVICE 4 Sheets-Sheet 1Filed Nov. 26, 1948 Oct. 23, 1951 D. F. WALKER 2,572,545

VARIABLE IMPEDANCE DEVICE Filed Nov. 26, 1948 4 Sheets-Sheet 2 Oct. 23,1951 D. F. WALKER VARIABLE IMPEDANCE DEVICE 4 Sheets-Sheet 5 Filed Nov.26, 1948 FIG. 60.

Oct. 23, 1951 WALKER 2,572,545

VARIABLE IMPEDANCE DEVICE Filed Nov. 26, 1948 '4 Sheets-Sheet 4 PatentedOct. 23, 1951 UNITED STATES PATENT OFFICE VARIABLE IMPEDANCE DEVICEDonald Ferguson Walker, Edinburgh, Scotland Application November 26,1948, Serial No. 62,169 In Great Britain November 28, 1947 11 Claims. 1

This invention relates to variable impedance devices of the typecomprising a closed or open chain of at least three serially-connectedimpedance elements energized by an input voltage applied acrossappropriate points of the chain'lf closed or across the ends of thechain if open, the chain being designed to yield a variation of outputvoltage which is related to the movement of a control in accordance withsome given law.

Impedances of this type are frequently usedfor example in electricalcomputers-but are liable to be rendered inaccurate by inaccuracies inthe mechanical control of the sliders (or like tapping devices) and, inthe case of wire-wound potentiometers, by the fact that as a sliderusually makes contact with only one point on each turn of wire theoutput voltage does not change smoothlybut in a series of steps each ofwhich represents the voltage drop across one turn. It is true that theheight of each step may be reduced-4hr example by increasing the numberof turns of the windingbut such procedure is often inconvenient toapply. These inaccuracies may be accentuated by the various expedientsusually resorted to in order to ensure approximate conformity with thegiven law, such as winding the wire on a specially shaped former, orwinding it linearly and moving the slider in accordance with the law bymeans of some mechanical contrivance.

The object of the invention is to provide a variable impedance device ofthe type stated of simple construction in which the law relating thevariation of output voltage to the movement of the control is observedto a high degree of accuracy.

In accordance with the present invention in a variable impedance deviceof the type stated the impedances of the said elements are such that theseveral total impedances between a datum point in said chain andsuccessive common points between said elements and between said datumpoint and the ends, if any, of said chain are in accordance with saidlaw, there being provided impedance components the impedance of each ofwhich is high compared with the impedance of any of said elements andthe number of which is greater than unity and less than the number ofsaid elements, tapping means for connecting one of said componentsacross any element and the remaining component or components across alike number of successive elements and/or off positions each to each,disposed on one side of said element, a common variable tapping meansadapted to engage two or more adjacent components successively, meansfor deriving said output voltage between said common variable tappingmeans and said datum point in said chain, and means actuated by saidcontrol (a) for transferring one of said remaining components toconnection across the adjacent element on the other side of saidelement, and (b) for traversing said common variable tapping means alongsaid one of said components whilst said component remains connectedacross said element, then, without substantial change of said outputvoltage, onto said one of said remaining components after transferencethereof, and then along said last-mentioned component whilst it remainsconnected across said adjacent element.

Also in accordance with the invention, in a variable impedance device ofthe type stated said elements comprise resistors, the resistances ofwhich are such that the several total resistances between a datum pointin said chain and successive common points between said elements andbetween said datum point and the ends, if any, of said chain are inaccordance with said law, there being provided two resistivepotentiometers the resistance of each of which is high compared with theresistance of any of said elements, tapping means for connecting one ofsaid potentiometers across any element and the other potentiometeracross an adjacent element or offposition, disposed on one side of saidelement, a common variable tapping adapted to engage said potentiometerssuccessively, means for deriving said output voltage between said commonvariable tapping and said datum point in said chain, and means actuatedby said control (a) for transferring said other potentiometer toconnection across the adjacent element on the other side of saidelement, and (b) for traversing a common variable tapping along said oneof said potentiometers whilst said potentiometer remains connectedacross said element, then, without substantial change of said outputvoltage, onto said other potentiometer after transference thereof, andthen along said other potentiometer whilst it remains connected acrosssaid adjacent element.

Compensation means may be provided for increasing the accuracy of saiddevice when said common variable tapping means is situated between theends of any one of said components or when said common variable tappingis situated between the ends of either of said resistive potentiometers,as the case may be.

In the accompanying drawings,

Figures '1 and 2a to 2e illustrate the principle of operation of theinvention,

Figure 3 is a curve showing a typical law observed by apparatus inaccordance with the invention,

Figure 4 shows diagrammatically compensation means,

Figure 5 shows an embodiment of the invention'in mechanically simplifiedform,

Figures 6a to illustrates the operation of the embodiment shown inFigure 5,

A simplified form of the invention will now be described in order tomake clear the principle of operation. The variable impedance devicechosen in this example follows a cosine'law between the angles and 90,so that in response to movements of the control spindle, e. g. oi; "apredictor, in dependence on some variable angle the output voltagebetween a slider and a datum point is always proportional to cos 0. Inthis ev ce-ri e Fi u e-17mm. mped e l m in the form oi resistors I to 9are connected in series to form an open chain between end points Aand J,across which points is applied an input voltage V. End point J, which isearthed, is the datum point 'referred to above. The eight common pointsbetween the resistors are B C, D resistor is thus connected betweenpoints A and B, resistor 2 between pointsB and C, and so on, resistor};being connected between I and J The resistances of the resistors aresuch that the voltages between the end point or common points andearthed end point J follow a cosine law with end point A representingzero degrees, common point B representing C representing and. so on,with earthed end point J representing 90f. The actual resistance of eachresistor is determined from the fact that the ratio of the totalresistance between B and J to that between A and J is cos 10, or 0.9848;the ratio of thetotal resistance between C and J to that between A and Jis cos 20, or 0.9397, and so on. V

4 v The end points and common points are brought out to tapping means inthe form of studs similarly designated A to J across certain pairs ofwhich may be connected the ends P and Q of a nrstresistive potentiometerII] the resistance of which is very high compared with that of any ofthe chain resistors.

Potenticmeter I0 is shown in Figure 1 connected across resistor 3, withthe ends P and Q of the potentiometer respectively engaging studs C andD It will be clear that when slider ll .of'potentiometer I0 is at theend P the potential of the slider, is the same as that of the point C sothat the output voltage V developed between the slider and the datum orearthed point J represents cos 20. When the slider is at the end Q theoutput voltage represents cos 30. And when the slider occupies someintermediate posipotentiometer I9 is moved to another pair of studs. Toavoid this interruption and ensure a smooth change of potentialthroughout the sca e a second component, in the form of a secondpotentiometer I2, is provided, exactly similar to the firstpotentiometer; the ends of the second potentiometer are R and S, and inFigure 1 it is shown connected across resistor 4 with ends R and Sengaging studs D and E Instead of each having a slider the twopotentiometers share 4 the same common variable tapping or slider H. Bya mechanical arrangement, hereinafter described, tliese "twopotentiometers always occupy adjacent pair of studs'that is, they arealways connected to three consecutive studs in the centre "one of whichis shared by both potentiometers.

In Figure 1 they are shown occupying consecutive studs C to E andsharing stud D control .effectinglthe movement of slider II independence on the angle 0 automatically ensures thatwhenthesliderreaches an end of a mqtentiomete l s potentiometer the otherpotentiometer is already in position across the next vacant resistorelement (if any) to receive it so that the slider can move .smoothlyfromthe one potentiometer to thefotl'ifwi'thout 'siibsta tiereaah'g'e 6fhiitput i e e 'Ui l sf i i f wh t 3 d i l s o d l qi i fi3" fl ie 1 r @l{6r th first -potentiomter to b trans erred to thene git vacant resistoriel ei'neiit beyond the isec; .bjii'd potentiometer and this the peteniqmetets and the slider continue their travel along the l enge .t p Thea ove angement will'be made'cleai'er .fri al i le me n re i e to P g ath l j oii e i nm i erai e' h w h the full range startihg from 0 andfinishing at 9Q", is tray The two potentiomete'fs areito sen c Figure v"d n jte' p esie first t l o lresistol s i| and 2; Yet the end A of theth 9 matine d 2 l ia sirl z is a e 99?- ie i t studs Ah-B5 aedjB. Q epeei relymeted, te'n i i q et r smear n a e n era i lfas hel e 'a g.i'li o sie flri is gene l es-th t i ete mfen P i -B Th commonslider I I;is, atthe end 1?; its put wa e wa er e n itepv-, lie nsaqw iioiedjbr'fqfihi qlii a ds th en th' r le qver i sthsiae' le be wee Wand 1 -i,reaclies this'end Q its; potential is that Dream 3 i iFii i f T e,Slider r i fi t sf i tallies p 'n mi' 'l ls fiur i-ie i s,pet ptiel niaiii i fi .et hat, iiiep im B during this process o'wing to the'closeprozgmity of -the end turns of thejtwo jpotentiometers.

gometer lfl is imposition-to receivelit, -As the slid r, c t nuin isrtrav rse, n ars th rend n ler y an e r d an over handf tothe studsD1,'E1. In'thiswaythe Towards the end of the scale represents cosor'zero. a Thefsystemworks infthe reverse'direc-tion in an e a s m laman r, 7

The precise moment in each hand-over-hand operation when a transferenceof a-potentiometer is effected is unimportant but itshould forconvenience'take place whilst the slider is moving iromthe midpoint oftheother potentiometertodsqne nd o t- A, s l

It willbe noticed that-the resistor elements may be divided into twogroupsthose across which The V potentiometer l0 may be connected andthose across which potentiometer 12 may be connected. Potentiometer l0may be connected across the odd numbered resistors I, 3, 5, I, 9 but notacross the even numbered resistors 2, 4, 6, 8, whilst the reverseapplies to potentiometer 12.

By lengthening the chain by another nine resistors beyond the earthedpoint J and by connecting the source or constant voltage across the endsof the lengthened chain the equipment may be adapted to cover the range0180 instead of 090. In this case the output voltage is of course thatbetween the slider and earthed point J, which remains the datum point,this voltage being negative when the slider is operating in the 90-180half of the chain.

In practice there are many more resistors in the chain than the ninewhich in order to simplify the explanation were assumedabove. The seriesof potential steps between successive common points are therefore manyand small, whilst the effect of a high resistance potentiometerconnected across a pair of adjacent common points is to smooth out thestep concerned into almost exact coincidence with the correspondingsection of the cosine curve relating output potential to sliderdisplacement. Moreover, no difiiculty is experienced in providing theseries of separate resistors each of which has a resistance that can bereadily determined from tables of cosines, instead of a resistor thatvaries along its length as in the prior art.

It Will be appreciated that the potential of the slider does not adhereto the cosine law with strict accuracy when the slider is at anyposition on a potentiometer other than at its ends. The potentialbetween these ends in fact follows a linear law and may be representedby a straight line joining the two adjacent plots on the cosine curvethat represent the potentials of the two common points across which thepotentiometer is connected. This is illustrated in Figure 3 which showsa cosine curve between 0 and 90 referenced in correspondence with Figurel, with the ends P, Q of potentiometer 10 connected to the points C, D.Where there are many resistors these plots lie close together and thestraight lines connecting the plots therefore lie very close to thecurve. Should however exceptional accuracy be required some form oferror compensation may be necessary. The design of such compensationmust allow for the facts that the extent of divergence of the straightline from the curve varies with the position on the curve of the plotsrepresenting the two common points concerned, being (in the case of acosine curve) most at 0 and least at 90, and that the error is such asto render the slider potential too low when at any position except theends. Any compensating potential must therefore vary with the positionof the resistor in the chain, must be zero when the slider is at eitherend of the potentiometer across it, and be additive when the slider isbetween those ends.

One example of a compensating circuit for a cosine potentiometer asabove described, the supply voltage of which is alternating, is shown inFigure 4. This incorporates a transformer 20 the primary 2| of which isconnected between earth and the higher potential end of thatpotentiometer with which the slider l l at any given moment is engaged,in this case potentiometer Ill. The secondary 22 of the transformer isconnected, in series with a very high resistance 23, between slider IIand the higher potential end of potentiometer II). If theresistance ofthe resistor to which the potentiometer is connected (resistor 3 in thiscase) is assumed zero in comparison with the resistance of potentiometer10 it will be seen that the secondary current creates a compensatingpotential difierence in the potentiometer only when slider I l is-not atone of the ends; when the slider is at the end P the secondary currentdoes not flow in potentiometer In at all and when the slider is at theend Q the secondary current is almost entirely diverted by resistor 3.Secondary 22 is connected in such sense that the compensating potentialincreases the potential of slider II in all other positions. It willalso be seen that the value of the compensating potential differencevaries with the position of the resistor in the chain, since primary 2|is connected to the potentiometer across this resistor.

In the above description it has been assumed that the equipment operatesin accordance with a cosine law. It may however operate in accordancewith other laws, for example a sine law or a square law, the necessaryalteration of the resistor values or of the compensating circuit will beobvious to those skilled in the art.

An embodiment of the invention will now be described by Way of exampleas applied to the cosine potentiometer form first described withreference to Figures 1 to 3, having as before an open chain of nineresistors and two potentiometers to cover the range 090. Thi isillustrated by Figure 5, in which details of mechanical constructionhave been much simplified in order to clarify the principle ofoperation. Reference letters and numerals corresponding to those ofFigures 1 to 3 are used where applicable.

A first contact bank is formed by mounting on the face of a firstinsulating disc 30 near the periphery five metallic contacts or sectors3| insulated from each other and equally spaced round the disc. Theycorrespond to alternate studs A C E G and 1 stops (not shown) areprovided between adjacent sectors A and I. A second contact bank isformed by mounting on a second disc 32, similar to the first disc, fivecontacts or sectors 33 corresponding to the remaining studs-B D F H andJ the stops (not shown) being between sectors B and J Discs 30 and 32are mounted in fixed relative coaxial position with the sector sides ofthe discs facing each other. The sectors of the two discs areinterconnected exteriorly by the appropriate resistors, e. g. sector Aon disc 33 is connected to sector 13 on disc 32 by resistor i, sector Bto sector C (on disc 30) by resistor 2 and so on. The remainingresistors are omitted from the drawing for simplicity. It will be seenthat as the total number of resistors is odd, the chain begins at theend contact or sector A of the first bank and ends on end contact orsector 5 of the second bank, the supply being applied as before acrossthese end sectors, of which sector J is earthed.

In between and coaxial with the discs is a ring 34 of insulatingmaterial bearing the toroidal windings of the two potentiometers l3 andI2; half the ring, circumferentially, comprises potentiometer Ill andthe other half potentiometer l2. Rotatably coaxial with the equipment sofar described is a single slider [I designed to traverse round the twopotentiometer windings. The ends P and Q of potentiometer ID areadjacent to the ends S and R respectively of potentiometer l2, the lastturn at each end of one potentiometer being so close to the last turn ofthe other potentiometer as to allow the slider to touch both togetherand thus to successive sectors.

allowthe slider'to pass from one pot nti meter tothe other without asudden change of poten tial, as described above. These turns are shownless closely spaced to avoid confusing the draw ing. Theends P and s ofthe potentiometer are led to two brushes P and S that engage the sec:tors of. disc 30 and the ends Q and R are ledto two brushes Q and R,engaging the sectors of disc 32. The brushes are fixed relative to thepotentiometer ring by means not shown androtate with it. The positionsof thehrliShcSrelative to each other and to the sectors Will appearlater. The brushes are not wide enough to bridge Slider H andotentiometer ring .34 are retated relative to the fixed discs by thefollowing system of gearing, in which provision ismade for theintroduction of a fixed angle sif desired, so that the output may beproportional to cos (E ia). insteadof to cos 0. Ahollow control shaft 40the angular position of which represents-the angle .0 passes freelythrough the centre of disc and operates afirst inputmember (not shown)of a differential gear 4| thesecond input member of which ,(also notshown) is operated :by a solid shaft d2 coaxially within shaft 4,3; theangular position of shaft '42 represents the fixed an le ,a. The outputfrom gear 4! is the shaft :43; this drives slider ll direct and, through4:11 reductiontgear 4d, the potentiometer ring 3d, a direction oppositeto slider .II by means of radial spokes .45. Shaft 42 may of course beset to zero if only .00 dis required. .It will be seen that as the divesectors are equally spaced round each disc the potentiometer ring 34rotates oneeiifth of a revolution :whilst each brush moves .fro itsposition on one sector to the corresponding position on the next sector.Owing ,to the reduction gear 4-4 the slider makes ,iourtths ofarrevolution during this period and will accord.- ingly make .onecomplete revolutionrelative to .the potentiometer ring, thus traversingboth .potentiometers.

In order to understand the operation of :this equipment, referenceshould be made .to Figurefi in which the electrical part of theequipment is shown in developed form with thesectors straight 7 insteadof curved and referenced %to correspond to Figure 1. It is convenientitO consider each sector as being hypothetically dividedcircumferentially into three parts (as sshown bv dotted lines)'labelledX, Y, andiZ in the directionfof the earthed end of the chain.The -.circumferential length of the insulated space between adjacentsectors is the same as that ofone part. :Each part secured to shaft 49,whichis then rotated inaccordance with the'variable input anglefi.:I;h,e

potential of slider H thus-varies in accordance with 605 (01a). Intraversing theiull range,-as before, from (0: ;a)=ll to (-0 a);=90f,-the position at the commencement is-shown in Fig ure oa.

.Slider H is at the end P of potentiometer v,lil, .which is engagingresistor l .by way-. of,brushes .P Q and sectors A B The positions "of.the brushes are, as follows: {P :and Q between parts X and Y ofsectorsA andBrespectively; R and S enteringxspaces J, B .and A Cfr0m-=$ectors J te se n eiivel :T Pos t v s lv hi R on se or J 'iwhiobsh wn a ai seemed the fi ure or onv n ence b ush S1 i -5W tor A at t issta e ha 1 elec ic ig cance; thei nos. en me ely o re po d to t p s tiono poten i m ter J2- The arm 1101; sho n? that carries brushes 5.1 and Q1is now against the stop, above referred to; between sectors B1 and J hesim a a m c y g th other rushe P and 1 now ain h s b w e sector A and IAWhilst he slider is tra e s n o enti met r ill from to Q a h b u h me ad ta e l to the len th o two pa t v n hi oe -i991 thebn sh s R1 an S movf 011 setter J nd A o eoiorsB and C espect l u ransferring potentiometer82 from the off position between sectors J a d bet en Which is noresister) into position across resistor 2 in the hand-over-hand mannerabove described; When therefore the slider leaves the end Q ofpotentiometerlll for the end B of potentiometer l2.,se,e Fig-ure;6b.thebrushes have reached the fol-lowing positions: P and Q on parts Z ofsectors A and B respectively'but about to leave them; and R and S onsectors B and ,C' re.- spectively, between parts X and Y. PotentiometerI2 is thus engaging resistor 2 when the slider moves onto it. This isthe position occupied bv the brushes in Figure 5 Whilst potentiometerflis engaged by the slider and its brushes are movingaalong sectors B andC the brushes P and Q of potentiometer It first traverse spaces AC and BD respectively and then reach parts X of sectors 0 and D (see Figure6c), thus transferring potentiometer iii to the next resistor 3 in thehand-over-hand manner. It will be seen that, as shown with reference .toFigure 1, pctentiometer I0 may ,be'connected across onlythe odd-numberedresis torsi. .e. those disposed horizontally in Figure 6whereaspotentiometer I2 may beconnected acrossonly the even-numbered;resistors,i, e. those disposedobliquely.

The rest of the operation follows closely that already described, untilthe end of the scale is reached with the slider at the end Q ofpotentiometer 10, connected across resistor 9, andwith potentiometer l2in an on position with its brushes onsectors J and A The essentialrequirements are that the brushes change see,- tors only when theirpotentiometer is not errgaged by the s1ider,-and, conversely, that aslong as apotentiometer is so engaged its brushes remain on theirrespective sectors. .So long as one potentiometer is in position across.the next resistor before the slider reaches the end of the otherpotentiometer .the precise relative positions of thebrushesisnot ofimportance.

The compensation circuit previously described withreference to Figure 4may easilybe applied to this embodiment, the only adaptation necessarybeingth e provision of meansfor effecting a connection toithe higherpotential end of whichever potentiometer isengaged by the slider. Asimple way of doing this is to'provide a commutatorifl (see Figure '7)havingtwo semiecircular segments ..,5l and 52 connected respectively toends P and Rof potentiometers In and l,.2'; the associatedbrush-rfic isganged with slider lll of potentiometers lfland 1-2 soas to rotate insynchronismwith it. The two gaps 5t, 55 between the'segmentsare solocated relative to brush 5 3 that whilst slider H is engagingpotentioneter -Il I,-brush.,53 v isv engaging, segment 5 so that the higherpotential end .P of this potentiometer is connected,to;brush,.53.by-wayof this segment.

Similarly whilst slider l i is engagin potential end I2, brush 53 isengaging segment 52, so that the higher potentiometer R of thispotentiometer is connected to brush 53. The connections from slider Hand brush 53 to the compensating transformer are as described withreference to Figure 4, the lead to the end P of potentiometer I0 in thatfigure being now connected instead to brush 53. The operation is thesame as before.

The above-described embodiments may be modified to cover the range 0-360instead of This may be efiected by employing only double the number ofresistors required for the 0-90 range and addin suflicient additionalsectors to raise the total number of sectors to equality with the totalnumber of resistors. This is shown in single diagrammatic form in Figure8, in which for simplicity the range 0 to 90 is covered by only threeresistors-referenced I, 2, and 3-instead of by the nine resistors of theabove embodiments. The part of the arrang ment covering this range isotherw se similar to Figure 1 and is correspondingly referenced. Tocover the range 90 to 180 anoth r three resi tors are added. each beingreferenced by the prim d number of the corresponding resist r in the 0to 90 range. To cover the range 180 to 360 (or 180 back to 0) no moreresistors are r ouired: the sectors of this range are cros -connected asshown to the sectors of the 0 to 180 ran e. The chain is now a closedone. The in t volta e is applied to the s ct rs at the 0 and 180 pointswhilst the 90 and 270 points are earth d and are the datum points. Theapplication oi this arrangement to the above-descri ed embodiment isquite strai ht orward: alt rnate sectors are located on disc 30 theremainder on d c 32. In t is arran ement continuous rotation of theotentiometer ring is of co rse permis i le and the stops and of!positions referred to above are not required.

The arrangement descr bed in the fore oing paragraph may be modifi d vey sim ly to o erate in accordance with a sine r ther than a cos ne law.This may be effect d merely by rotating the calibrations (see Figure 8)clockwise through 90, leaving the connections to the s p ly and to earthas they were before. The e rthed datum points of zero output nowcorrespond to 0 and. 180.

The above embodiments are describ d in order to illustrate the in entionand are not intended to limit its scone, which embraces variousmodifications of the eq i ment that sho ld be readily apparent. Theresistors may for instance be replaced by other forms of im edanceelements, for exam le chokes: the impedance components orpotentiom'eters may be in the form of tap ed autotransformers; there maybe more than two com onents, though of course their n mber must be atleast one less than the number of impedance elements so as to leave atleast one unbridged element for one of the components to be transferredto, this component being any of the ones not engaged by the commonvariable tapping at the time. The tapping means may be in the form of acommon variable tapping traversing all components as described above, ormay take the form of a separate tapping for each component combined witha simple selector switch that transfers the output lead from connectionto the tapping of one component at the end of the traverse of thattapping along that component, to connection to the tapping of the nextcomponent. The compensation circuit described for application to avariable impedance device operates under a sine or cosine law may bemodified in detail; for example the connection from the unearthed end ofthe primary of the transformer may be made to the lower potential end ofthe potentiometer engaged by the slider rather than to the higherpotential end, and the connection from the secondary may be made to theend of this potentiometer that is not connected to the primary. Moreoverthe mechanical part of the control may be of a different type from thatdescribed.

What I claim is:

1. A variable impedance device comprising in combination, at least threeimpedance elements connected in series and having impedances satisfyinga predetermined law; two impedance components each having a highimpedance in comparison with the impedance of any of said elements;tapping means for connecting said components in shunt to any twoconsecutive ones of said elements, respectively; a tapping memberadapted to engage either of said components; means for changing theshunt connection of said component not engaged by said tapping memberfrom one element adjacent the other shunted element to the other elementadjacent thereto; and means for traversing said tapping member along anyof said components while said component is connected in shunt to one ofsaid elements.

2. A variable impedance device comprising in combination, at least threeimpedance elements connected in series and having impedances satisfyinga predetermined law; means for applying a voltage to the seriesconnection of said elements; two impedance components each having a highimpedance in comparison with the impedance of any of said elements;tapping means for connecting said components in shunt to anytwoconsecutive ones of said elements, respect vely; a tapping memberadapted to engage either of said components; means for changing theshunt connection of said component not engaged by said tapping memberfrom one element adjacent the other shunted element to the other elementad acent thereto; means for traversing said tapping member along any ofsaid components while said component is connected in shunt to one ofsaid elements; means connected to sa d tapping member for compensatingthe error of the device when said tapping member engages a point betweenthe ends of any of said components; and means for deriving a voltagebetween said tapping member and a predetermined point of the seriesconnection of said elements.

3. A variable impedance device comprising in combination, at least threeresistor elements connected in series and having resistances satisfyinga predetermined law; two resistive potentiometers each having a highresistance in comparison with the resistance of any of said elements;tapping means for connecting said potentiometers in shunt to any twoconsecutive ones of said elements, respectively; a tapping memberadapted to engage either of said potentiometers; means for changing theshunt connection of said potentiometer not engaged by said tappingmember from one element adjacent the other shunted element to the otherelement adjacent thereto; and means for traversing said tapping memberalong any of said potentiometers while said potentiometer is connectedin shunt to one of said elements.

' 4. A variable impedance device comprising in 7 Combin o t least threeresistor elements-conping member adapted to engage either of saidnotentiometers; means for chan in the shunt connection of saidpotentiometer not. en a ed b said tapping member from one elementadjacent to the other shunted element to the other element adjacentthereto; means for traversing said tapping member along any of saidpotentiometers while said potentiometer is connectedjin shunt to one ofsaid elements; and means for doriving a voltage between said tappingmember and a predetermined point of the series connection of saidelements.

5. A variable impedance device comprising in combination, at least threeresistor elements connected in series and having resistaneessatisfying apredetermined law; means for applying a voltage to the series connectionof said elements; two resistive potentiometers each having a highresistance in comparison with the resistance of any of said elements;tapping means for connecting said potentiometers in shunt to any twoconsecutive ones of said elements, respectively;

a tapping member adapted to engage either of said potentiometers; meansfor changing the shunt connection of said potentiometer not encaged bysaid tapping member from one element adjacent the other shunted element.to the other element adjacent thereto; means for traversing said tappingmember along any of; said potentiometers while said potentiometer isconnected in shunt to one of said elements; means connected to saidtapping member for compensating the error of the device when saidtapping member engages a point between the ends of any of saidpotentiometers; and means for derivinga voltage between said tappingmember and a predetermined pointof the series connection of saidelements.

6. A variable impedance device comprising in combination, a luralityofresistance elements connected in series and including odd numberedelements and even numbered elements, said elements'having resistancessatisfying a predetermined law; means for applying a. voltage to theseries connection of, said elements; two banks of contacts, thesuccessive junctions of said re: sistor elements being connected tosuccessive contacts in each of said banks alternately; two pairs ofbrushes each being arranged for cooperation with oneof said banks,respeotivelyr Said brushes'being adapted to engage the contacts of saidbanks without bridging any two contacts; a first resistivepotentiometer; a second resistive potentiometer, said potentiometerseach having a high resistance in comparison with the resistance of'anyof said elements; a, connection between one of said brushes cooperatingwith one of said banks and one end of said first potentiometer; aconnection with one of said brushes cooperating with the. other of saidbanks and the i numbered elements and said even numbered-elements; atapping member adapted to engage either. of said potentiometers; meansfor changing the shunt connection of said potentiometer not engaged bysaid tapping member from one element adjacent the other shunted elementto the other element adjacent thereto; means for traversing said tappingmember along any ofsaid potentiometers while said potentiometer isconnected in shunt to one of said elements; and means for deriving avoltage between said tapping member and a predetermined-point of theseries connection of said elements '7, A variable'impedance devicecomprising in combination, a plurality of resistance elements connectedin series and including odd numbered elements and even numberedelements, said elements having resistances satisfying a prcdeter minedlaw; means for applying a voltage to the series connection of saidelements; two banks of contacts, the successive junctions of saidresistor elements being connected to successive contacts in each of saidbanks alternately; two pairs of brushes each being arranged forcooperationwith one of said banks, respectively, said brushes be ng p edto eng ge the contacts of said banks without bridging any two contacts;a first resistive potentiometer; a second-resistive potentiometer, saidpotentiometers being disposed end-to-end on a circle and each, having ahigh resistance in comparison with the resistance of any of saidelements; a connection between one of saidbrushes cooperating with oneof said banks and one end of said first potentiometer;ia connection withone, of said brushes cooperatin with the other of; said banks and theother end of i t p tentiometer; connectionsbetween the ends of saidsecond potentiometer and the remaining ones of said brushes, saidbrushes being disposed in relation to said banks tor said potentiometersbeing adapted to bridge, respe.o tively, consecutive ones of said oddnumbered elements and said even numbered elements,- each pair ofadjacent ends of said potentiometers being connected to those of saidbrushes which engage contacts of the same of said banks; a tappingmember being adapted to rotate about the center of the circle and toengage either of said potentiometers; means. for changing the shuntconnection of said potentiometer not en gaged by said tapping memberfrom-one element adjacent the other shunted element to the other elementadjacent thereto; means for traversing said tapping member along any ofsaid potentiometers while said potentiometer connected in shunt to oneofsaid elements, whereby said-tape ing member upon leaving one of saidpotenti ometers, engages the other of said potentiometers; and means forderiving a voltage between said tapping'member and a predetermined pointof the series connection of said elements,

8. A Variable impedance device comprising in combination, a plurality ofresistance elements connected in seriesaand including odd numberedelements and even. numbered elements, said e1E: ments having resistancessatisfying a predetermined law; means for applying a voltage to theseries connection of said elements; two banks; of contacts, thesuccessivejunotions of said reasistor-elements being connected tosuccessive c cts in. ach of aid b ks. a ternately; two pairs of brusheseach being arranged for cooperation with onset said banks, respectively;said brushes being adapted to engage the contacts of said ksw thout bridin y two c tacts; a i s 13 resistive potentiometer; a second resistivepotentiometer, said potentiometers being disposed end-to-end on a circleand each having a high resistance in comparison with the resistance ofany of said elements; a connection between one of said brushescooperating with one of said banks and one end of said firstpotentiometer; a con- ,nection with one of said brushes cooperating withthe other of said banks and the other end of said first potentiometer;connections between the ends of said second potentiometer and theremaining ones of said brushes,. said brushes being disposed in relationto said banks for said potentiometers being adapted to bridge,respectively, consecutive ones of said odd numbered elements and saideven numbered elements; a tapping member being adapted to rotate aboutthe center of the circle and to engage either of said potentiometers;gearing means associated with said tapping member and said brushes andensuring that said tapping member traverses the entire length of one ofsaid potentiometers while said brushes connected to said tapping memberremain on the same of said contacts of said banks interconnected by aresistor element, and prior to the transference of said tapping memberto the other of said potentiometers said brushes connected to the otherof said potentiometer-s are transferred to the pair of contactsinterconnected by the adjacent resistor element; and means for derivinga voltage between said tapping member and a predetermined point of theseries connection of said elements.

9. A variable impedance device comprising in combination, a plurality ofresistance elements connected in series and including odd numberedelements and even numbered elements, said elements having resistancessatisfying a predetermined law; means for applying an alternatingvoltage to the series connection of said elements; two banks ofcontacts, the successive junctions of said resistor elements beingconnected to successive contacts in each of said banks alternately; twopairs of brushes each being arranged for cooperation with one of saidbanks, respectively, said brushes being adapted to engage the contactsof said banks without bridging any two contacts; a first resistivepotentiometer; a second resistive potentiometer, said potentiometersbeing disposed end-to-end on a circle and each having a high resistancein comparison with the resistance of any of said elements; a connectionbetween one of said brushes cooperating with one of said banks and oneend of said first potentiometer; a connection with one of said brushescooperating with the other of said banks and the other end of said firstpotentiometer; connections between the ends of said second potentiometerand the remaining ones of said brushes, said brushes being disposed inrelation to said banks for said potentiometers being adapted to bridge,respectively, consecutive ones of said odd numbered elements and saideven numbered elements; a tapping member being adapted to rotate aboutthe center of the circle and to engage either of said potentiometers;gearing means associated with said tapping member and said brushes andensuring that said tapping member traverses the entire length of one ofsaid potentiometers while said brushes connected to said tapping memberremain on the same of said contacts of said banks interconnected by aresistor element, and prior to the transference of said tapping memberto the other of said potentiometer-s said brushes connected to the otherof said potentiometers are transferred to the pair of contactsinterconnected by the adjacent resistor element; a transformer includinga primary and a secondary, said primar being connected between apredetermined point of the series connection of said resistor elementsand either end of said potentiometer being traversed by said tappingmember, said secondary being connected between said tapping member andeither end of said potentiometer being traversed by said tapping member;and means for deriving a voltage between said tapping member and analternating predetermined point of the series connection of saidelements, whereby the voltage derived between said tapping member andthe predetermined point of the series connection of said elements iscorrected when said tapping member is located between the ends of saidpotentiometer.

10. A variable impedance device as claimed in claim 9, a commutatorhaving a first semicircular segment and a second semi-circular segment,said first semi-circular segment being connected between said primaryand said potentiometer being traversed by said tapping member, saidother semi-circular segment being connected to the other potentiometer;and a brush connected to said primary and traversing said commutator insynchronism with said tapping member whereby said brush always engagessaid segment connected to said potentiometer engaged by said tappingmember.

11. A variable impedance device as claimed in claim 10, the connectionbetween said secondary and said potentiometer being completed by way ofsaid commutator.

DONALD FERGUSON WALKER.

REFERENCES CITED The following references are of record in the me ofthis patent:

UNITED STATES PATENTS

